Dalton s Atomic Theory. 367 



thousands of vapour densities had been taken, it reappeared in 

 the form of an empirical law. 



Chemists have often wondered why Dalton did not acknow- 

 ledge as correct the observations of Gay-Lussac that gases 

 always combine in simple volumetric proportions. Various 

 improbable or absurd reasons* have been assigned for his 

 reluctance to adopt the results of the distinguished French 

 chemist. The real reason is near at hand. Dalton could 

 not accept Gay-Lussac's law of the combination of gases in 

 simple volumetric proportions, because, if he had done so, he 

 would have abandoned the chief principle of his atomic theory. 

 I will illustrate this assertion by an example. The first 

 principle of Dalton is, that when two elements form only 

 one compound, the compound is a binary one unless some 

 cause appears to the contrary. 



Only one compound of hydrogen and oxygen and only 

 one compound of hydrogen and nitrogen were known, hence 

 the formula of the first would be HO and of the second HN. 

 Nitric oxide, on account of its low specific gravity, was like- 

 wise considered to be a binary compound of the formula NO. 

 (N.S. 317.) Water, according to Gay-Lussac, is a compound 

 of one volume of oxygen and two volumes of hydrogen. If wo 

 assume one volume of hydrogen to contain 1000 atoms 

 (molecules), then, if Dal ton's formula HO is right, one 

 volume of oxygen must contain 2000 atoms. 



Nitric oxide is formed, according to Gay-Lussac, of one 

 volume of nitrogen and one volume of oxygen. Therefore, 

 if Dalton's formula NO is correct, one volume of nitrogen, 

 like one volume of oxygen, must contain 2000 atoms. Hence 

 we have : — 



one volume of hydrogen 1000 atoms, 

 one volume of oxygen 2000 „ 

 one volume of nitrogen 2000 „ 



Now Gay-Lussac asserts that ammonia is a compound of one 

 volume of nitrogen and three volumes of hydrogen — 

 one volume of nitrogen 2000 atoms, 

 three volumes of hydrogen 3000 „ 



and its formula would be N 2 H 3 if Gay-Lussac's observations 

 are correct. 



Dalton's principle requires the formula NH. Hence the 

 alternative presented itself to Dalton either to reject his first 

 principle or Gay-Lussac/s observation. As the atomic theory 



* R. 47. Grundziige der theoretischen Chemie von Lothar Meyer. 1890, 

 p. 30. 



